In the field of coding data in a transmitted signal, the transmitted signal may comprise multiple sub-periods with orthogonal frequencies. Data coding methods of this type are also known as “Orthogonal Frequency Division Multiplex” (OFDM) schemes, where the signal is composed of a superposition of signals corresponding to a set of sub-carriers, wherein the sub-carriers have mutually orthogonal frequencies. The frequencies are orthogonal in the sense that the time dependent functions on a support corresponding to the signal duration of the transmitted signal are orthogonal functions in the mathematical sense. The signal duration is therefore inversely proportional to the spacing of neighboring sub-carriers.
One fundamental problem that is inherent to all orthogonal frequency division multiplex systems is the high Peak-to-Average Power-Ratio (PAPR). Since the transmitted signal is a superposition of many sinusoidal signals, the time domain samples are approximately gaussian distributed, which gives rise to strong peaks. This is a problem because power amplifiers are linear only within the limited range of input power. Stronger peaks are clipped to the maximum output power. Hence, either the signal is distorted by the non-linear clipping operation or the power amplifier must operate with low average power so that even peaks are still within the linear range. This, however, leads to a poor efficiency and over-dimensioned amplifiers. It is therefore important to employ algorithms that reduce the peak to average power ratio.
The second characteristic of orthogonal frequency division multiplex signals is that the Power Spectral Density (PSD) of the transmitted signal decays quite slowly with f−2 outside the allocated frequency range. This is due to the spectral components of the rectangular pulse corresponding to the symbol duration. The Out-of-Band-Power (OBP) therefore interferes with systems transmitting on neighboring frequency bands. Therefore, in order to reduce mutual interference between two independent systems to an acceptable level, the systems must be separated by a rather large guard interval in frequency domain. This guard interval reduces the overall spectral efficiency. In the following, we will use the term Out-of-Band-Power (OBP) for the power that is contained in the power spectral density outside the assigned frequency range. If the Out-of-Band-Power is reduced, then the guard interval between the systems could be reduced and the spectral efficiency could be increased.
Due to the importance of Peak-to-Average-Power-Ratio reduction, many algorithms have been proposed in literature. Many of these algorithms, however, suffer from the fact that side information needs to be transmitted to the receiver. For instance, one could employ several different interleavers, each of which giving rise to a different time domain signal. Then the signal with the lower Peak-to-Average-Power-Ratio is chosen. The receiver of cause needs to know which interleaver was used. Not only leads the additional transmission of the side information to the decrease of spectral efficiency, but this data is very important because an error in the side information can lead to a complete loss of the orthogonal frequency division multiplex symbol.
Another Peak-to-Average-Power-Ratio reduction algorithm is proposed by Tellado in his PHD theses (Tellado, “Peak-to-Average-Power Reduction for Multi-carrier Modulation”). This algorithm does not suffer from the latter drawback. He proposes to reserve a set of sub-carriers for the sole purpose of Peak-to-Average-Power-Ratio reduction: these carriers, called Reserved Tones (RT), are referred to as reserved sub-carriers in the following. The reserved sub-carriers do not carry any data, but are modulated such that the Peak-to-Average-Power-Ratio of the resulting time signal is minimized. This method yields of course a reduction of spectral efficiency, but since the reserved sub-carriers do not carry any data, the system is not vulnerable to loss of side information. In fact, if the channel is slowly varying and the transmitter has channel state information, the reserved sub-carriers could be scheduled on sub-carriers that are in a deep fade, where data transmission would be impossible anyway. The sub-carrier reservation method has found its way into the WIMAX standard, which shows that this algorithm is indeed of practical interest.
For the problem of Out-of-Band-Power-Reduction, there is also a technique proposed by Brandes (Brandes, Cosovic, Schnell, “Sidelobe Suppression in OFDM Systems by Insertion of Cancellation Carriers”), which is based on reserved tones. These tones are sub-carriers, in the aforementioned paper called Cancellation Carriers (CC) are located at the edges of the spectrum and are modulated such that their spectra approximately cancel the spectra of the data carriers, resulting in the reduction of Out-of-Band-Power of the total orthogonal frequency division multiplex signal.
The above described problems occur in particular in OFDM(A) based Systems. Such Systems are suffering in mobile scenarios uplink connections, by the imperfect time synchronization of the different allocated chunks. The multitude of mobile terminals have slight but notable frequency offsets, which violates the orthogonal nature of the OFDMA signal at the base station receiver.
Typical examples for OFDM(A) based uplink system concepts are the IEEE 802.16 standard (WiMax), 802.11g, or proprietary concepts like Flash-OFDM.
The multitone modulation itself puts a significant requirement on the power amplifier design, namely in the need of a back off, which reduces the PA efficiency, or achievable output power. For both problems individual solutions already exist to reduce the unwanted effects, by using cancellation carriers or reserved tones. However the cancellation carriers are dedicated for a single use to day, to achieve the wanted effect.
OFDM Modulation puts a significant requirement on the power amplifier design, namely in form of a back off, which reduces the PA efficiency, or achievable output power.
Peak to average power ratio (PAPR) is a measure which describes the required linearity of a transmitter to transmit signals with multi-code or multi-tone characteristic and is also dependent on the used modulation schemes. The aim is to generate a signal w/o significant distortion, therefore avoiding degradations in the Signal-to-Noise-Ratio (SNIR) required at the receiver.
Therefore, the PAPR describes the needed linearity especially of the power amplifier to achieve a good link performance. The linearity is achieved by introducing a back off from the PA saturation point, commonly known as the 1 dB compression point, about a certain amount of dB. The magnitude of this back off depends on the used modulation scheme and the multi-code or multi tone characteristic of the system concept. The back off improves the linearity of the PA on the cost of higher power consumption, or a reduced maximum transmission power.
From a mobile terminal point of view, large power back offs are reducing the achievable range, or if the range is maintained the PA linearity causes a high power consumption and shorten the battery life time significantly. This effect on increased transmitter design complexity and power demand mounts up in system concepts which are utilizing balanced Multiple Input Multiple Output (MIMO) concepts as e.g. 2×2 MIMO, which requires the presence of two transmitters in the terminal. Complex transmitter designs and high power consumption are duplicated.
Systems with OFDMA multi carrier transmission are suffering on the PAPR effects in the uplink. Especially if mobile terminals are being used the effect on the battery life time is negative. There are concepts to reduce the PAPR by cancellation carriers which are using reserved tones. Tone reservation dedicates specific subcarriers to be used for something different than the user data transport. Usually, the user data is mapped to a plurality of subcarriers and is segmented into time intervals, comprising resource block units.
Tone reservation is performed in such a way that additional subcarriers are added to the resource block (or subcarriers of a resource block are being removed from the MAC as carriers bearing user data and getting a reserved status), where the reserved tones have a modulation which does not depend on user data. The modulation is controlled by the output signal and if the peak power is going to exceed a certain threshold, the modulation of the reserved tones is chosen in such a manner that the PAPR is decreased. Depending on the implemented complexity, PAPR reductions of about 3 dB and higher can be achieved, which in terms of power consumption and achievable range is a big step.
The associated disadvantage is that the spectral efficiency is lowered because of the reservation of subcarriers with the only purpose of PAPR reduction. To utilize the result of the PAPR reduction in the cell range, the improved performance is used to increase the average power i.e. utilization of the amplifier closer to the saturation point. The known method [Brandes, S., Cosovic, I., Schnell, M., “Sidelobe Suppression in OFDM Systems by Insertion of Cancellation Carriers”] to combat these effects is also a tone reservation, however the modulation of the reserved subcarriers is performed in such a manner that the spectral mask gets steeper and the power spill over into adjacent resource blocks is reduced by up to 20 dB. The procedure can be feedback based: using the input data, the resulting output spectrum is processed to derive the parameters for the modulation of the reserved tones. An alternative implementation is a statistical approach producing modulation patterns which are reducing the constructive addition of the tones.
The associated disadvantage of this method is an increase of the PAPR. However, as long as the range is not a point, and spectral efficiency and protection of adjacent bands and resource blocks is the objective, the method is very successful. The side lobe suppression shows best performance when the cancellation carriers are allocated at the chunks edges as the most outer tones.
Today methods are known to reduce PAPR or to reduce adjacent channel power by tone reservation, however exclusively for the one or the other problem, with the result that for each optimization the position of the cancellation carriers should be different.